Electrical Distribution Systems CIRCUIT BREAKERSHigh-VoltageCircuitBreakersHigh-voltage circuit breakers (including breakers rated at intermediate voltage) are used forservice on circuits with voltage ratings higher than 600 volts. Standard voltage ratings for thesecircuit breakers are from 4,160 to 765,000 volts and three-phase interrupting ratings of 50,000to 50,000,000 kVA.In the early stages of electrical system development, the major portion of high-voltage circuitbreakers were oil circuit breakers. However, magnetic and compressed-air type air circuitbreakers have been developed and are in use today.The magnetic air circuit breaker is rated up to 750,000 kVA at 13,800 volts. This type of circuitbreaker interrupts in air between two separable contacts with the aid of magnetic blowout coils.As the current-carrying contacts separate during a fault condition, the arc is drawn outhorizontally and transferred to a set of arcing contacts. Simultaneously, the blowout coil providesa magnetic field to draw the arc upward into the arc chutes. The arc, aided by the blowout coilmagnetic field and thermal effects, accelerates upward into the arc chute, where it is elongatedand divided into many small segments.The construction of this type of circuit breaker is similar to that of a large air circuit breaker usedfor low-voltage applications, except that they are all electrically operated.Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressedair directed toward the separable contacts of the breaker to interrupt the arc formed when thebreaker is opened. Air-blast circuit breakers have recently been developed for use in extrahigh-voltage applications with standard ratings up to 765,000 volts.Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil. Currentinterruption takes place in oil which cools the arc developed and thereby quenches the arc. Thepoles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltagecircuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers arenormally sealed. The electrical connections between the contacts and external circuits are madethrough porcelain bushings.CircuitBreakerControlAs we have discussed, circuit breakers may be remotely operated. In order to operate thebreakers from a remote location, there must be an electrical control circuit incorporated. Figure7 shows a simple control circuit for a remotely-operated breaker.Control power is supplied by an AC source and then rectified to DC. The major components ofa simple control circuit are: the rectifier unit, the closing relay, the closing coil, the tripping coil,the auxiliary contacts, and the circuit breaker control switch.Rev. 0 Page 11 ES-15